1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a layer containing an organic compound, and to a method of manufacturing the light emitting device. In this specification, the term light emitting device includes an image display device, a light emitting device and a light source (including illuminating device). Also, the following modules are included in the definition of the light emitting device: a module obtained by attaching to a light emitting element a connector such as an FPC (flexible printed circuit; terminal portion), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) system.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting member and are characterized by their thinness and light weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich an layer containing an organic compound, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the layer containing an organic compound to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is increased, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
Organic compounds for forming an layer containing an organic compound (strictly speaking, light emitting layer), which is the center of a light emitting element, are classified into low molecular weight materials and polymeric (polymer) materials. Both types of materials are being studied but polymeric materials are the ones that are attracting attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
Known methods for forming these organic compounds into films are, for example, an evaporation method, a spin coating method, and an ink jet method. The spin coating and the ink jet are particularly well known as methods that allow a light emitting device to display a full-color image using a polymeric material.
However, when the spin coating is used, the organic compounds are formed over an entire film forming surface and therefore, it is difficult to selectively form the film in which the organic compound is formed only at a portion intended to form the film and the film is not formed at the portion which is not necessary to form the film.
Further, the active matrix light emitting device is formed with a wiring for inputting an electric signal from an external power source to a driver circuit formed above a substrate, and a wiring for electrically connecting a light emitting element comprising a cathode, an anode and an layer containing an organic compound formed by an organic compound formed at a pixel portion with the external power source and therefore, when the organic compound is formed at the portions (terminal portions) of the wirings connected to the external power source, there poses a problem that ohmic contact cannot be achieved with external power source. Especially, when the layer containing an organic compound is formed by the spin coating method, it is difficult to lead out electrodes (cathode or anode) formed on the layer containing an organic compound to the terminal portions.
Therefore, the present invention provides a method for selectively forming a polymeric layer containing an organic compound and a connection structure for electrically connecting an electrode (a cathode or an anode) provided on the layer containing an organic compound to a wiring extending from a terminal portion.
An ink jet method, known as a method for selectively forming a polymeric organic compound film, is advantageous in that different organic compounds respectively emitting light of three colors (R, G, B) can be separately applied at a time. However, this method is disadvantageous in its poor film formation accuracy, and therefore it is difficult to control the formation of films so as to obtain the uniformity of the film. Thus, the obtained film is likely to have unevenness. The reasons for such unevenness in the ink jet method are, for example, unevenness in nozzle pitch, unevenness due to the ink jetted in a curved manner, low stage matching accuracy, a timing lag between the ink discharge and the stage movement, and the like. For example, this ink jet method has problems in implementation conditions such as a clogged nozzle for ink jet due to internal viscosity resistance of an ink produced by dissolving an organic compound in a solvent, an ink jetted out from a nozzle which fails to reach a desired position, as well as problems for practical use such as increased cost due to the necessity of a dedicated apparatus having a high precision stage or an automatic alignment mechanism, an ink head and the like. Moreover, since the ink is spread after reaching, a margin is required to a certain degree as a space between adjacent pixels. As a result, it becomes difficult to provide higher definition.
Therefore, in an active-matrix light emitting device using a polymeric organic compound, the present invention has an object of providing a method for selectively forming a polymeric material layer, which is simpler than in the case where the ink jet method is used; the present invention has another object of easily forming a structure in which an layer containing an organic compound is not formed at a junction of a wiring to be connected to an external power source.
A light emitting device conventionally has a problem in that external light (light exterior to the light emitting device) incident on non-light emitting pixels is reflected by a bottom face of a cathode (a face on the side being in contact with a light emitting layer), resulting in the bottom face of the cathode acting as a mirror. As a result, the exterior landscape is disadvantageously reflected on a viewing face (a surface facing a viewer side). In order to avoid this problem, it is devised that a circular polarization film is attached on a viewing face of the light emitting device so that the exterior landscape is not reflected on the viewing face. However, since the circular polarization film is extremely expensive, the manufacturing cost is disadvantageously increased.